An image forming apparatus, such as a copying machine, a printer and a facsimile, which employs an electrophotographic printing method has been conventionally known. The image forming apparatus which employs the electrophotographic printing method forms an electrostatic latent image on a surface of an image bearing member having a photoreceptor layer. The image forming apparatus develops this electrostatic latent image into a toner image with use of toner supplied from a developing device, transfers the toner image on a sheet such as a paper, and then fixes the toner image on the sheet.
The developing device using a two component developer (hereinafter referred to simply as a developer, too) including toner and carrier stirs and circulate the developer in the device. Thereby, the toner and the carrier are rubbed together and triboelectrically charged. At a developing stage, the thus triboelectrically charged toner move, by electrostatic suction, to the electrostatic latent image formed on the image bearing member, and forms a toner image.
In recent years, there have been demands for speeding-up of image formation. This necessitates the toner to be precisely and speedily charged to a predetermined electric potential. In order to precisely and speedily charge the toner to a predetermined electric potential, it is necessary to sufficiently rub the toner and the carrier while the developer is stirred and carried.
For that purpose, Patent Document 1 discloses an arrangement in which a developer carrying member having a spiral wing body is provided on a developer carrying path, for circulating and carrying the developer, which is provided in a developer container for containing a two component developer, and a mesh screen member is provided between the wing bodies of the developer carrying member.
According to this arrangement, the developer is not only carried by rotation of the developer carrying member, but also stirred and passed through the mesh screen member several times, thereby appropriately charging the toner electrically by friction between the toner and the carrier. The Patent Document 1 describes that the toner can be given appropriate amount of charging by immediately dispersing the supplied toner in the developer.
[Patent Document 1]
Japanese Unexamined Patent Application Publication Tokukaihei No. 10-63081 (published on Mar. 6, 1998)
However, even the developing device described in the Patent Document 1 is insufficient for an image forming apparatus which is more improved in speed. Thus, this developing device cause an unevenness in toner density and toner electric charges. Therefore, the developing device cannot secure high developing performance in such an image forming apparatus.
That is, in the arrangement of the developing device of the Patent Document 1, the supplied toner is dispersed in the developing device by passing through the screen member together with the preexisted developer in accordance with stirring movement of a developer stirring member. Therefore, the more the screen members are provided, the more screen members the toner passes through. This allows an improvement in dispersing effect. Therefore, this makes it possible for the toner to be uniformly dispersed and speedily charged to a desired electric potential.
However, today in an image forming apparatus, there are strong demands for downsizing in addition to speeding-up. Therefore, there is a limit in improving the dispersing effect by increasing the number of screen member. This necessitates development of a developing device having a simple arrangement in which the unevenness in toner density and toner electric charges is not caused, and the stable toner can be speedily supplied in developing.